Conveyer mechanism



R. E. J. NQRDQULST CONVEYER MECHANISM Filed Nov., 18, M3259 2 Sheets-Sheet 1 IN Yr-: TOR if SYM 1Q.

ATToR E'Ys D- 15, 1942. R. E. J. NoRDQulsT CONVEYER MECHANI SM Filed NOV. 18, 1939 2 Sheets-Simcf;

AYTCSRNEYK Patented Dec. 15, 1942 UNITED STATES PATENT oFFlcE coNvEYEa mcnANIsM Ronald E. J. Nordquilt, Maplewood, N. J., assignor to American Can Company, New York, N. Y.,

a corporation of New Jersey Application November 18, 1939, Serial No. 305,182 9 Claims. (Cl. 'I4-241) The present invention relates to a belt conveyer mechanism or the like and has particular reference to devices for keeping the conveyer in a centralized position on the pulleys over which it erable by a slight deviation of the conveyer from` its proper path of travel. v

Numerous other objects and advantages of the invention will be apparent as it is better understood from the following description, which, taken in connection with the accompanying drawings,

discloses a preferred embodiment thereof.

Referring to the drawings: Figure 1 is a side elevation of a conveyer mech- 'anism -embodying th`e instant invention. with Darts broken away;

Fig. 2 is a horizontal section taken substantially along the broken line 2-2 in Fig. 1; with parts broken away; F18. 3 is an enlarged vertical transverse section taken substantially along the broken line 2 3 in Fig. 2, parts being broken away;

Fig. 4 is an enlarged fragmentary front view taken substantially along a vertical plane indicated by the line 4-4 in Fig. 2; and

Fig. 5 is a wiring diagram of the electric apparatus usedl with the mechanism.

To exemplify a preferred form oi the instant invention, the drawings illustrate the principal parts of an endless belt conveyer of a character which is in general used in many commercial installations where articles, during manufacture, are conveyed over comparatively long distances within buildings."I Such a conveyer preferably includes a metallic belt which is set up to carry sheets of tinplate from which cans or containers are produced. The belt is preferably wider than the sheets so that no portions of the latter extend over the edges of the conveyer. For such a use the conveyer will usually run approximately thirty six inches or over in widthand it is this great width together with the metallic properties of the belt which gives it a tendency to run off to one side of the pulleys over which it passes.

ings illustrate a stainless steel endless conveyer belt I I (Fig. l) which takes over a driving pulley I2 and over an idler pulley I3 which are mounted in a main frame i4. The driving pulley I2 is carried on a driving shaft i8 which is journaled in bearing blocks l1 secured to brackets I8 which are formed on the main frame I4. The driving shaft may be rotated in any suitable manner, such as, by an electric motor indicated by numeral i! in the wiring diagram of- Fig, 5.

The idler pulley i3 is mounted in a manner which permits of changing the position of its axis so that the pulley may be shifted by swinging it through an arc relative to the frame i4 and this may create a pressure on one side or the other and .effective along the edges of the conveyer to move it laterally and thus to keep it in a straight line jpath. For this purpose the idler pulley I3 is As an example of such a conveyer, the drawmounted on an idler shaft 2| which is carried in bearings 22 of a horizontal movable carriage or swivel yoke 23 having'side arms 24 (see also Figs, 2 and 3). The ends of the idler shaft extend beyond the bearings 22 and carry rollers 25 which ride on smooth finished surfaces 24 formed on the tops of extending brackets 21 projecting out from the main frame I4.

The swivel yoke 23 is slidably mounted on a horizontal swivel plate 3l so that it may be shifted relative to the plate through an arc having its center, indicated by the letter X (Fig. 2). located midway of the breadth of the idler pulley Il at its axis. For this purpose the swivel yoke is formed with an arcuate rear edge 32. This arcuate edge of the yoke operates in an arcuate groove Il which is prOvided in an upright Shoulder 34 formed on the swivel plate 3|. The yolfe is re- .tainedl against vertical displacement by shouldered cap screws 21 winch operate in slots 3l formed in the yoke. The screws extend through the yoke and are secured in the plate.

Movement of yoke 23 relative to the swivel plate Il is effected by a reversible electric motor 4| which is actuated in a manner which will be hereinafter explained in connection with the wiring diagram in Fig. 5. The motor is secured to a bracket 42 which is formed as a part of the swivel plate. The motor shaft, indicated by the numeral 43,` carries a sprocket 44 which drives an endless chain 45. The chain also takes over a sprocket 46 which turns on the threaded 'shank 41 of an eye bolt 48. The shank oi the eye bolt is looselycarried in bearings'll of a bifurcated swivel bracket 5| having a depending stem 52 which is pivotally secured in a lug 5I of the swivel plate 3|, The head end of the eye bolt 48 is the swivel `yoke 2l.

Hence when the motor rotates, it revolves the sprocket 4I on the eye bolt shank 41 and this rotation shifts the bolt sidewise in its bearings Il. Such a shitting movement oi the eye bolt slides the swivel yoke 2l in an arc on the swivel plate Il, said yoke being controlled in its arcuate iloating movement by its engagement with the opstanding shoulder 34 on the swivel plate 3|, as best seen in Fig. 2. The turning of the yoke thus shifts the idler pulley |3 and its axis and brings pressure on the adjacent edge, which may be designated the running-oil' edge, of the conveyer. Hence, such shift in the axis position of the pulley tends to bring the conveyer belt beck into a centralized location on the pulley.

The motor being reversible in character makes it possible to slide the swivel yoke in eltherof two sidewise directions so that the pulley can be likewise shifted. Thus a correcting pressure may be exerted on either edge of the conveyer for returning it from either side to its central running position on the pulleys.

To make such a correction on the proper side of the conveyer, the motor is controlled by a pair of normally open electric limit switches 51, Il (Fig. 2) which are secured to the frame extension brackets 21, one on each side of the conveyer adjacent its outer edges. A pair oi' normally closed stop switches 59, 6B (Fig. 4) are also disposed adjacent the limit switches for stopping the conveyer motor I! in case the conveyer runs too far oil' the pulley il. The operation of these switches will be explained hereinaiter in connection with the wiring diagram.

Provision is made for maintaining the conveyer in a taut condition at all times. For this purpose the swivel plate 3| is movably carried on a support bracket 8| (Fig. 3) which is secured to a cross beam l! oi the frame H. The swivel plate is lor-med with a depending tongue 63 which operates in .a longitudinal slideway Il provided in the top oi the bracket. Hence the swivel plate and the machine parts carried thereon may be shifted longitudinally of the machine so that the idler pulley il may be forced back against the conveyer and this pulls the conveyer taut.

The pulley is maintained in this outward taut belt position by a compression spring S5 (Figs, 1 and 2) whichls interposed between the motor bracket l! of the swivel plate 3| and the cross beam 82 of the frame. A holt 86 which is surrounded by the spring is provided to restrict the outward movement of the swivel plete h1 case the conveyer should break or to hold the pulley when the conveyer is not in piece. The inner end of the bolt is secured swivel plate while the opposite or cuter exten-.is through a boss 61 formed in the cross beam 82. Locknuts Il are provided on outer end of the bolt to limit its spring position.

Reference should now be had to the wiring diagram in Fig. 5. Electric energy the machine motor l! and for the various control circuits is preferably supplied from s. gers erator 1| whereas the energy for operating the conveyer correcting motor il preferably plied from a separate source of energy such as a generator 12.

When starting the machine from a standstill a starting circuit A is used which circuit inw clude-s the motor Il, the two normally closed stop switches 5I, il, a manually operated aus closed stop switch 14, and a similar 2,304,843 matinal on s pivot pm u which is threaded into normally open start switch 1l. These devices are all connected in series by suitable wires. At one end oi' this series oi connected devices the motor i9 is connected by a wire 8| to a main lead wire 82 which connects with one side oi the generator 1|. At the other end of the se ries of devices the start switch 15 is connected by a wire 83 to a main return wire Il which connects with the other side of the generator 1|.

Hence when the press button starting switch 15 is momentarily closed the energy from the generator 1| travels along the lead wire 02, wire 8|, the motor Il and the series of switches 5I, GU, 14, 15 connected thereto, the wire Il, and returns to the generator by way of the return wire u. This excites the motor Il and thus sets the machine in operation.

A holding circuit B is provided to maintain the motor l! in operation after the press button starting switch is released or opened. This holding circuit includes a wire which connects with one side of a solenoid l1 oi a relay switch 88. The other side of the solenoid is connected by a wire Si to the wire which connects the motor I8 to the stop switch 5l. Energy traveling along the holding circuit while the start switch 1l is still closed energizes the solenoid t1 and thus closes the relay switch Il.

Closing of the relay switch II establishes l. machine operating circuit C of which the holding circuit is a part and which cuts out the starting switch 15. Establishment of this circuit permits the motor Il to continue in operation after the starting switch is released or opened. I'his circuit includes a wire 9| which connects one side of the switch Il to the wire between the stop switch 1I and the start switch l5. The other side oi the switch Il is connected by a wire 92 to the return lead wire I4. Hence while switch 88 is closed, energy passing through the motor I! and the stop switches Il, to, 1l, is returned to the generator by way of wire sl, switch 8l, wire 82, and return wire M.

Manual stopping or the machine when desired v alter it has once been started may be effected by opening the stop switch 1l. This breaks the holding circuit B and the operating circuit C Just described and thus stops the motor II. The machine is also stopped when the conveyer runs of! too tar on either side of the pulley. When this happens the conveyer belt edge engages and opens one or the other oi' the stop switches Il, B0. Opening of either of these switches also breaks the holding circuit and the operating circuit and hence stops the motor Il.

Correction of the conveyer through the limit switches l1, il when it tends to run oi! to one side or the other of the pulley is brought about by certain circuits which are established and broken in timed sequence and over predetermined periods in order to allow time for an adjustment lto become effective before another adjustment is made. An explanation oi such circuits is best brought out by assuming the conveyer to be running oi! in a given direction, as for example, to the righi ss viewed in ng. 4. conveyer' will then engage and close the normally open limit switch Il. A

The limit switch 5l is included in an energizing circuit D which comprises a wire 9| which connects one side o! a solenoid relay Il to the main lead wire I2 o! generator 1|. The other side o! the solenoid is connected by wires 01, Il to the limit switch 58. The limit switch is also connected by e wire l! to a normally closed cam operated tim- The edge of the ing switch |30 c! a timing device |0I. The timing switch is also connected by a wire |32 to the return lead wire 34 of generator 1 I. Hence when the limit switch 53 is closed energy from the generator 1I travels along this energizing circuit D and energizes the solenoid relay Il. Energizing of the solenoid relay 98 closes three switches |35, |33 and |01.

A correction motor circuit G is immediately established by closing oi the switch |01. This sets into motion the reversible motor 4| in a direction which will swing the swivel yoke 23 so that the belt will be caused to move back and away from the switch 58 now being considered.

At the same time a timing circuit E is established by closing of the switch |05 and electrical energy thereupon sets into motion the timing device III. I

Closing of the switch |05 at this time does not result in immediate establishment of a circuit, but does prepare for what will later be a holding circuit F. This latter circuit will be established as the direct result of closing o! a switch member in the timing device |I. The timing device operates through a definite time cycle which per mits the yoke actuating motor 4| to operate an exact number of seconds. 'Ihis timing device thereupon holds the yoke in its adjusted position a given number of seconds. This tends to give the conveyer belt time to make its adjustment on the pulley. f

Following such holding of the yoke, further shifting may take place in the same direction if the belt has not moved far enough to release the switch 50. Where this happens the timing device |0| again operates to give another deflnite sluiting movement followed by another rest period. This continues as long as the switch 53 remains closed.

Should the belt after any one of these adjust ment periods have moved back far enough to re- Ieaae the switch 53, the timing device |0| will not be again operated by the timing through the switch |05. y

However, should the belt following any oi' the aforesaid adjustment periods actuate the switch I1 on the otherslde, similar circuits will be established as Just described, but in this case the motor 4| will be operated in reverse. This will be later more fully described.

The result of such control is that when the belt adjustments are being made the shifting ai the yoke and the holding of the yoke after each shifting period take piace during a definite time nterval and this is determined by the timing device I0|. The various switch elements of this :iming device will be later described, but first :he circuits E, F and G will be further explained.

The timing circuit E includes wires II2 which connect the main lead wire 32 of gener- |.tor 1| to the relay switch |05. The relay switch s also connected by a wire ||3 to a timing moor I I4 of the timing device |0|. The timing mov.cris also connected by a wire ||5 to the gen- :rotor return wire 84. Hence electrical energy .raveling along the circuit excites the timing moor ||4 and sets the timing device |0| in operition immediately after the limit switch 53 has een closed by the lateral shifting ory running off t the conveyer.

The timing device |0I includes a rotatable lhai't II1 which is revolved by the timing motor I4 and which carries three cams H3, IIS, |20. ihese cams respectively operate the switch |00 ni circuit E passing open timing switches |23, .|24 of the respective circuits E and F, these switches being operated at predetermined times after the timing device is set in motion as will now be explained.

Setting in motion of the timing device I0| immediately closes the timing switch |24 of the holding circuit F and opens timing switch |00 of the energizing circuit D and thereby establishes the holding circuit. It is such opening of the timing switch |30 that breaks the energized circuit D and thereby renders the limit switch 30 ineffective for the following cycle of the timing device This allows time for the yoke actuating motor 4| to be set in motion and to bring about a shifting of the pulley and an adjustment of the conveyer belt. In such holding circuit F energy from generator 1|, traveling through' the solenoid relay 38, then continues along wire 31, a wire |23, closed relay switch |00, wires |21, |23, closed timing switch |24, a wire |29, and return wire 34, back to the generator. The timing switch |24 remains closed for a predetermined length of time and is then opened by its rotating cam |20.

During the period that the timing switch |24 is closed the correction mot-or circuit G established by the closing bf switch |01, functions to excite the correction motor 4| in th'e proper direction to bring about an adjustment of the conveyer belt as hereinbefore explained. The source of energy for this circuit Gis supplied from the auxiliary generator 12 which is provided with three phase lead lines I3I, |32, |33.

When switch |01 is closed the energy from the auxiliary generator 12 travels along lead line I3 i, a wire |35, a solenoid |33 of a relay switch |31,

wire |33, switch |01, a wire |33 returning by way of wire |33 to the generator; This current energizes the solenoid |36 and thus closes the relay switch |31.

Closing of the relay switch |31 connects the auxiliary generator lead wires |3I, |32, |33 with motor lead wires |4I, |42, |43 which extend directly to the correction motor 4|. Hence energy from the generator is conducted direct into the motor. This connection is maintained for only a. short time, preferably only a few turns of the motor, and is then broken to permit the adjustment thus made on the conveyer to become enective.

Breaking of the correction motor circuit G is brought about when the timing switch |24 opens as hereinbefore mentioned. The opening of this switch also breaks the holding circuit F and hence permits the switches |05, |03, |01 to open. This action breaks the other remaining circuits sc that the entire system will be in readiness for a further adjustment of the conveyer when such is he energizing circuit D and two other normally 2o necessary. y

I'he results of such anadjustment are not immediately effective. hence provision is made for delaying further adjustment until a predetermined period oi' time has lapsed so that the conveyer belt will have -time to locate itself on the newly adjusted pulley. This delay is brought about by the timing device III which is kept in operation through a delay circuit H which is established by the closing of timing switch |23 by its cam ||3 just prior to the breaking of the holding circuit F.

When timing switch |23 is closed, energy from the generator 1| travels from the lead wire 32, along a wire |45, through timing switch |23, a wire |43, wires ||3, III, 34 by way of the timing @uch a motor ||4 and back tothe generator.

closed circuit keeps the timing motor in operation. When the cam Il! opens the switch |23 and thus cuts of! the source of energy the timing motor stops.

Just prior to stopping of the timing motor, however, the timing cam ||l closes the timing switchI l" of the energizing circuit D. The cam operated timing switches |23, |24 are both left open while switch l is left closed. This completes the entire cycle of the timing device and the system of circuits are again ready for a subsequent adjustment of the correction motor 4|.

The description of the wiring diagram thus far has taken into account only those circuits which are required to turn the reversible correction motor 4| in the proper direction when the conveyer tends to run on toward the right in Fig. 4. A similar set of circuits are provided to turn the motor 4| in the opposite direction when the conveyer tends to run off toward the left in Fig. 4. At that time the conveyer operates the limit switch 51.

'Ihe limit switch 51 is included in an energizing circuit K which is identical with the energizing circuit D and is severed by the same timing switch IBI. When the limit switch l1 is closed this circuit receives its electrical energy from the generator 1|, along the wire the current passing through a solenoid relay IBI, wires |52, |53, switch l1 and a wire I which connects into the wire Sl of the circuit D. This energizes the solenoid relay |l| and thus closes three other switches |53, |51, I.

Switch |56 is connected by wires ICI, |82 to the respective wires H2, I Il of the timing circuit E so that the timing device |l| will be set in motion when the switch Ill is closed. Switch |i1 is connected on one side to the wire |53 of the energizing circuit K and on the other side by a wire I which connects with'th'e wire |21 of the holding circuit F. -Hence the closing of switch |51 establishes a holding circuit L which is identical with the holding circuit P and which maintains the solenoid relay Ill energized when the limit switch 51 and the timing switchilil are closed.

Switch IBI is connected on one side by wires |66, |61 to the lead wire |33 of the auxiliary generator 12r The other side of the switch is connected by a wire |88 to a solenoid |68 of a relay switch |1|. The solenoid is also connected by a wire |12 to the lead wire lll of the auxiliary generator. Hence the closing of switch |58 establishes a correcting motor circuit M which is identical to the motor circuit C+ and which energizes the solenoid I" and thereby closes the relay |1|. Closing of lthe relay causes the current to travel from the auxiliary generator 12 along the wires |81, |12 and a third wire |13, directly into the correcting motor 4| by way of connecting lead wires |15, |10, |11.

The periods of operationA and the breaking of these circuits K, L and M are the same as for the circuits D, F and G and are controlled in the same manner by the timing device It is therefore thought that the detailed operation of the timing device in connection with these circuits can here be omitted for the sake of brevity.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacricing all of its material advantages, the form hereinbefore described being merely a preferred embodiment thereof.

I claim:

l. A control device for maintaining proper running alignment of a conveyer belt on its pulleys, comprising in combination with such conveyer belt. a swivel yoke for supporting one of said conveyer pulleys, said yoke having a curved wall, a swivel plate for movably holding said swivel yoke and its supported pulley and having a curved seat engaging the curved wall of said yoke, a threaded eye bolt engaging said swivel yoke and adapted to exert pressure on the same for shifting said yoke with its pulley from one side to the other by sliding engagement with said curved support, a sprocket carried on said threaded eye bolt and movable thereon to effect such shifting, a reversible electric motor geared to and adapted to rotate said sprocket to cause said pulley to exert an edge pressure on one side-oi said belt, and electrical means engageable with said conveyer belt and operable when the conveyer belt veers to one side of its regular path of travel for actuating said motor to rotate said sprocket in a direction to apply said pulley edge pressure to the proper side of the belt to return the belt toits centralized running position on said pulleys, said electrical means being in electrical circuit with said reversible motor.

2. A control device for maintaining proper running alignment of a conveyer belt on its pulleys, comprising in combination with such a conveyer belt, a support carriage for one oi the conveyer pulleys said carriage being movable through an arc having 4its axis disposed substantially centrally of the longitudinal axis of said pulley, a main electric motor for actuating said conveyer, an auxiliary reversible electric motor for moving said carriage in either direction, a pair of electric detector switches disposed adjacent the path of travel and on opposi-te sides of the conveyer belt, a said switch being engaged by said belt when the belt veers from its central running position and moves toward that side. means electrically connectedl between each oi said switches and said auxiliary motor for selectively actuating the motor in a direction to properly shift the said carriage to exert an edge pressure on said belt to return the belt to its centralized running position on said pulleys, a pair of stop switches disposed adjacent the path and on opposite sides oi travel of the conveyer belt. said stop switches being located outside of said detector switches, a said stop switch being engaged by said belt if the latter moves too far out oi' its central position on that side, and a switch controlled holding circuit in electrical circuit with a said stop switch and said main electric motor for terminating operation of the motor and for stopping the conveyer when said stop switch is actuated.

3. A control device for maintaining proper running alignment of a conveyer belt on its pulleys, comprising in combination with such a conveyer belt, a movable support carriage for one of the conveyer pulleys, an auxiliary electric motor for moving said carriage and its supported pulley to one side or the other, a pair of electric detector switches located one on each side of said belt, each switch being adapted to establish an electric circuit between the switch and said motor when the belt moves from its central running position and engages such a switch, said switche.l being adapted to actuate said motor in opposite directions to move said carriage and its supporter pulley on an axis disposed substantially centrally of the longitudinal axis of said pulley, whereby to exert pressure on said belt to return the same to its centralized running posi-tion on said pulleys, and a timing device disposed in electrical circuit with said switch circuit for delaying said motor actuation to permit an adjustment to become effective before a subsequent adjustment is made.

4. A control device for maintaining proper running alignment of a conv eyer belt on its pulleys, comprising in combination with said conveyer belt, a support, a carriage having sliding movement on said support and in which one of the conveyer pulleys is journaled, means on said support engaging an edge portion of said carriage for guiding the same in an arcuate path of travel, and electrically controlled means actuated by and operable when the conveyer belt veers to one side oi its regular path of travel for slidably shifting said carriage and i-ts pulley relative to said support through said arcuate path of travel on an axis disposed substantially centrally ofthe longitudinal axis of said pulley, whereby said supported pulley exerts an edge pressure on said belt to return the same to its centralized running position on said pulleys.

5. A control device for maintaining proper running alignment of a conveyer beit on its pulleys, comprising in combination with said conveyer belt, a support, a carriage having sliding movement on said support and in which one ofthe conveyer pulleys is journaled, means on said support engaging an edge portion of said carriage for guiding the same in an arcuate path of travel, a detector element located at a side of said belt and engageable by an edge of the belt when the belt veers from its centralized running path, and electromechanical means in electrical circuit with and operable by said detector element when so engaged for slidably shifting said carriage and its pulley relative to said support through said arcuate path of travel on an axis disposed substantially centrally of the longitudinal axis of said pulley, whereby the pulley exerts an edge pressure on said beltto return the same -to its centralized running position on said pulleys. r

6. A control device for maintaining proper running alignment of a conveyer belt on its pul' leys,V comprising in combination with said conveyer belt, a support, a freely floating carriage movable on said support and in which 'one oi' lthe conveyer pulleys is journaled, means on said support engaging said carriage for guiding the same in an arcuate path of travel, means for holding said carriage under spring tension in a horizontal direction to maintain said belt in taut running condition while permitting movement of said carriage through an arcuate path of travel for varying the edge pressure on dierent sides of the belt, and means for bodily moving said carriage relative to said support along said arcuate path of travel and with it the pulley carried thereby when said belt veers from its centralized running path, so that edge pressure exerted on laid belt serves to return it to its centraliled running position on seid pulleys.

7. A control device for maintaining proper running alignment of a conveyer belt on its pulleys, comprising in combination with said ccnveyer belt, a support plate, a swivel yoke member constituting a carriage mounting for one of the conveyer pulleys supported for free floating movement on said plate, an arcuate shoulder on said support plate engaging an arcuate portion of said yoke member for guiding the latter in an arcuate path of travel, said yoke member including horizontally spaced side arms in which said conveyer pulley is journaled, and means disposed adjacent and actuated by said belt and operable when the belt runs` to one side of its central path of travel for bodily shifting said swivel yoke and its supported pulley on said plate4 through an arcuate path oi' travel, so that the pulley carried by said yoke exerts an edge pres- -sure on said belt to return the same to its centralized running position 'on said pulleys.

8. A control device for maintaining proper running alignment of a conveyer belt on its pulleys, comprising in combination with said conveyer belt, a horizontally disposed freely n oating swivel yoke for supporting 4one of said conveyer pulleys and having a curved wall portion, a horizontally disposed Swivel plate for freely supporting said swivel yoke and having a curved seat disposed in engagement with said curved wall portion oi' said yoke, and means actuated by and operable when the conveyer belt `veers to one side of its regular path of travel for bodily shifting said swivel yoke with its supported pulley relative to and upon said swivel plate by sliding engagement of the yoke with said curved plate seat, so that the said pulley exerts an edge pressure on said belt to return the same to its centralized running position on said pulleys.

9. A control device for maintaining proper running alignment of a conveyer belt on its puileys, comprising in combination with such a conveyer belt, a support plate including an arcuate .upstandingshoulder having a seating groove therein, a carriage in which one of the conveyer pulleys is iournaled, said carriage having a curved portion seated within said groove and freely movable on said support plate through an arcuate path of travel on an axis disposed substantially centrally of the longitudinal axis of said pulley, a reversible electric motor for moving said carriage in opposite directions on said support plate through said arcuate path of travel, a pair of electric detector switches disposed adjacent the path of travel ci the conveyer belt on opposite sides of the same, one of said switches being engaged by said belt when the belt veers from its central running position and moves toward that side, and independent electro-mechanical means electrically oonnected between each of said switches and said motor for selectively actuating the motor in a direction to properly shift the said carriage to exert an edge pressure on said belt to return the belt to its centralized 

